Butter can either be made with fermented or fresh cream. The fermentation process consists of taking a certain agent such as cream and breaking it down into a simpler form. This same process is used in the creation of alcohol. You can buy butter that is fermented or non-fermented.
Those who care about having one versus the other will definitely want to look into which type they are picking up at the supermarket, because chances are you can find it on the label. The more research you do into finding out which types of butter are fermented and which are not, the more likely you will be to find the kind you want.